The present invention relates in general to new and improved drivers and in particular to a pneumatically operated driver for driving fasteners into hard structures such as steel plate or concrete, or steel plate overlying concrete.
Pneumatic drivers are well known in the art as shown for example by U.S. Pat. No. 3,952,398. In essence, a compressed gas is caused to act on a piston to drive a hammer against the work surface. Such devices have proven to embody advantages not found in more conventional drivers which employ explosive charges in place of a compressed gas. Among the salient advantages of pneumatic drivers over conventional drivers is the precision with which the stroke and the impact force of the hammer can be controlled.
Since they first came into use, pneumatic drivers have undergone a series of transformations. One of the significant improvements that have been made relates to a safety mechanism which prevents the driver from operating when the trigger is pressed unless the foot of the tool bears against a firm surface. The implementation of this safety feature requires the use of a special control valve selectively adapted to vent the interior of the driver cylinder to atmosphere, as disclosed in my copending U.S. patent application Ser. No. 637,571, filed Dec. 4, 1975 and now U.S. Pat. No. 4,040,554 for Pneumatic Apparatus.
In my application Ser. No. 637,571 such venting occurs through the piston cushion positioned on the interior surface of the end wall of the cylinder and, in part, through the end wall itself. The latter construction is subject to a number of disadvantages, not the least of which is the fact that it requires that the length of the driver be increased by substantially the entire length of the control valve. Such an increase in size not only makes the hand-held tool less convenient to use, but also adds to its weight. Further, such an arrangement requires relatively complex machining of the end wall, particularly if an angled conduit for the gas is employed as must be the case when the increase of the overall length of the driver is to be kept within bounds. This additional machining increases the cost of manufacture of the entire equipment.
Such costs are further increased by the necessity for providing an hermetic seal between the reciprocating hammer and the end plate. In my Application Ser. No. 637,571, the seal is positioned below the interior surface of the end plate and is no longer easily accessible once the piston cushion is installed. If the cushion is to be anchored in the end plate, it is necessary to increase the thickness of the latter in order to keep the seal below the cushion. Here again the length and weight of the driver must be increased to accomodate such an arrangement. In addition, the manufacture of the end plate as well as the installation of the seal, become relatively complex and expensive operations.
Prior art cushions in themselves incorporate certain disadvantages. It must be remembered that the purpose of the cushion in a hammer driver that is provided with a proper safety mechanism is to cushion the impact of the piston against the end wall. In the device disclosed in my copending application Ser. No. 637,571, such impact occurs only under certain conditions, specifically when the driver is used to drive fasteners into material that does not have the requisite hardness. Under normal operating conditions, i.e. when the driver is used with material of the proper hardness, the recoil which occurs when the work surface is struck disables the driver and arrests the power stroke of the piston at a predetermined spacing from the cylinder end wall. The aforesaid spacing is determined by the length of the hammer which is chosen to cause the piston, during normal operation, to make contact with the cushion without materially compressing the latter.
In prior art drivers the piston cushion is fastened to the interior surface of the end wall by means of a bonding agent. It has been observed that there is a tendency for the piston cushion to separate from the end wall upon continued use. It is believed that this may be due to the penetration of high pressure air between the cushion and the end wall. Alternatively it may be due to the inability of the bonding to withstand the compression of the cushion followed by an upward force of short duration exerted by the partial vacuum that is created as the piston withdraws. Partial or complete tearing of the bond may occur, which will result in a loss of operating efficiency and which may, in extreme cases, disable the driver.
Further, where venting to atmosphere occurs through a duct in the cushion and in the end wall, cushion wear will tend to occur at the entry to the duct due to the fact that the air in the cylinder is at a relatively high pressure, e.g. 150-175 psi. When the piston descends in its power stroke, wear tends to occur at the duct as the air in the cylinder is rapidly vented through this duct in the cushion. Further wear tends to occur when the piston ascends and air rushes in the opposite direction through the same duct and into the cylinder.
The problem of providing a cushion which will not separate from the end wall, which will wear well and facilitates provision for a sliding seal for the hammer, is compounded by the need to assure adequate venting of the cylinder. If the volume of air under the descending piston during the power stroke of the latter is not vented fast enough, a back pressure will be built up which will slow down the descent of the piston and detract from the force of the impact that can be delivered by the hammer. Thus, the efficiency of the driver may be impaired.
Some prior design efforts in this area have taken the form of using a cushion that covers only the central portion of the end wall of the cylindrical wall of the latter. Such a cushion has a number of disadvantages, primary among which is the damage that may be inflicted on the piston when it descends on the cushion at full force. Since the piston impacting on such a cushion is supported only by the relatively small area which coaxially surrounds the hammer, flexing of the non-supported piston portion may occur upon impact and this can result in permanent damage to the piston. A similar possibility of damage exists if the cushion is designed to be engaged by only a limited area of the piston at its periphery.
A further problem of prior art pneumatic drivers resides in the difficulty of providing access to all parts of the driver. For example, if damage to the piston cushion occurs, it is advantageous if the end plate is readily removable for purposes of repair of replacement. In many pneumatic drivers of the prior art such removal is carried out only with difficulty and in some devices the end plate is permanently fastened to the cylinder. In the latter case, access to the interior of the cylinder is poslsible only through the top portion of the cylinder, which requires the removal of the piston and of the blocking structure above the piston.
In any hammer driver which is in active service, the hammer requires service or replacement from time to time. In some pneumatic hammer drivers the hammer is fastened to the piston in a manner where both must be replaced if one becomes defective. In other prior art drivers the hammer extends through a bore which penetrates the piston and it is fastened to the latter by a nut threaded onto the hammer outside the cylinder. Quite apart from its propensity to loosen under vibration, this type of construction is also prone to leakage through the pistonpenetrating bore. Further, delays are often incurred during the re-assembly of such prior art equipment due to the difficulty of precisely aligning the hammer with the bore in the foot portion of the tool.